First, I was going to use algae instead of benthic mud. Blue green algae can be anaerobic and I have tested it. It does produce some voltage. However, instead of measuring the voltage after thirty days, I was thinking of measuring bacteria density using the voltage. There would be a 100%, 75%, 50%, 25%, and 0% composition of algae to water ratio. I would use the voltage I got from the algae and plot a graph. Hypothetically, the graph would go down as less algae is put in. However, after the 100% and 75%, I found the voltage actually went up. Now, I am really confused about the rest of my experiment and how it will turn out. I have only done the 100% and 75% so far. I did 3 trials, however none of them turned out consistent. Trial 1 had 25mV while Trial 2 had 29mV and Trial 3 had 60mV. This may have happened because I used graphite epoxy for some and conductive epoxy for some. Also, when I used 0%(tap water), I found that there was it was 10mV.

Should I switch back to benthic mud or continue my experiment to see how it turns out? Do you have any suggestions to improve consistency so the graph will turn out better? Do you know why the voltage went up from 100% to 75% when I diluted the 75% with tap water? Should I use distilled water instead?

Blue green algae work well in a microbial fuel cell. However, I think it would be difficult to measure the microorganisms directly. In a MFC, the microorganisms actually form a biofilm on the surface of the electrode and this is necessary before voltage is produced. It is difficult to measure microorganisms that are densely packed in a biofilm.

The current produced depends on the rate of electrons moving from the bacteria as they use the anode electrode as a terminal electron receptor for energy production. So the current is a reflection of the overall metabolism of the microorganisms, and this can change with a change in temperature, oxygen conditions, sunlight or food availability. It's probably going to be difficult to maintain a high voltage at all times and get really nice reproducible graphs.

Since V = IR, (Ohm's law) you could increase the voltage if you reduced the resistance in the system. What is the concentration of ions in your tap water? What type of ions are present? It sounds like the tap water decreased the resistance in the cell. Distilled water would probably increase the resistance and so would probably decrease the voltage. It could be that adding tap water rinses out waste products that are inhibiting cell metabolism. Can you think of any other possibilities?

I think you have done a great experiment here with the tap water. I recommend repeating the tap water results to see if they are reproducible and doing more background reading to see if you can determine what the tap water is doing in your system. However, your project is due soon, so you should really concentrate on writing up the results for your display board.

I used distilled water and it seemed to be somewhat reproducible. The 100% 50% 25% were all proportional almost exactly. However, the 75% seemed to be lower in voltage than the 50% for two of the trials. It may have been because we used different test solution for the trials.(We had limited algae, so we used the same algae for 100%, 50%, and 25% first by taking half of the solution out then filling the rest with distilled water, then doing 75% separately. The electrode was taken out of the water, and placed in 75% composition. Every time do electrode was in oxygen, the voltage seemed to drop significantly). The third trial, we started with 75% first for the different % compositions and the results found to be very reproducible. When we took the electrode out this time, we soaked it in 100% algae while exchanging test substances.

We also found that the electrodes were extremely inconsistent. The average was around 30mV; however, some electrodes we found produced 80mV while others produced only 2mV. Do you think the inconsistency of the electrodes is why the tap water was so inconsistent or do you think we should just soak the electrode in the 100% composition?

What kind of electrodes are you using? You are getting proportional results for the dilutions, except for two of the trials, but the problem may be related to the electrode itself, or to the connection between the electrode and the rest of the cell. I'm not sure I can explain your results.

You may need to do more experiments to find out what is happening, and this may be beyond the scope of your project. If your project is due, you should write up your results, and you can explain what additional experiments you would do if you had time. Or, maybe you have a topic to investigate for next year's project.

We used conductive epoxy from Radio Shack for all of them with carbon cloth. Some produced very unexpected results while some may not have been functioning. The connection was fine. We used the same setup for all of the trials. I may repeat the experiment a few more times to verify the results.